Linear load cell bridge including a non-linear strain sensitive element

ABSTRACT

To compensate for non-linearities in a bridge circuit comprising a plurality of strain gauge transducers arranged to provide an output signal proportional to the load on a member, a non-linear strain sensitive element is mounted in the area of the member sensitive to load strain and electrically connected in series in one arm of the bridge. A matching non-linear strain sensitive element is mounted in an area of the member not sensitive to load strain and is connected in an adjacent arm of the bridge to compensate for temperature-dependent non-linearities.

United States Patent [191 Farr " [111 3,759,093 Sept. 18, 1973 LINEARLOAD CELL BRIDGE INCLUDING A NON-LINEAR STRAIN SENSITIVE ELEMENT [76]Inventor: Emory w, Farr, 830 Del cono Ave.,

West Covina, Calif. 91791 [22] Filed: Nov. 10, 1971 211 App]. No.:197,413

521 US. Cl .L 73/141 A 51 Int. Cl. G01] 5/18 [58] Field of Search73/885, l4l A; 324/57, 62; 323/75, 75 B [56] References Cited UNITEDSTATES PATENTS 3,085,193 4/1963 Perino 323/75 B x 3,178,938 4/1965 Rugel/l966 Ormond 73/885 R X- 3,245,252 4/1966 First et al 73/88.5 R

Primary Examiner-Richard C. Queisser Assistant Examiner--Stephen A.Kreitman Attorney-Christensen & Sanborn [57] ABSTRACT To compensate fornon-linearities in a bridge circuit comprising a plurality of straingauge transducers arranged to provide an output signal proportional tothe load on a member, a non-linear strain sensitive element is mountedin the area of the member sensitive to load strain and electricallyconnected in series in one arm of the bridge. A matching non-linearstrain sensitive element is mounted in an area of the member notsensitive to load strain and is connected in an adjacent arm of thebridge to compensate for temperature-dependent non-linearities. v

8 Claims, 2 Drawing Figures Patented Sept. 18, 1973 3,759,093

LINEAR LOAD CELL BRIDGE INCLUDING A NON-LINEAR STRAIN SENSITIVE ELEMENTFIELD OF THE INVENTION This invention generally relates to load cells,and, more particularly, to a strain gauge bridge therefor whose outputvaries linearly with applied load.

BACKGROUND OF THE INVENTION A load cell of a column type constructioncomprises a deformable member which is placed in an effective positionto sense an applied force or'load. Included are a plurality of straingauge transducers which are disposed on the deformable member so as tosense tension and compression forces therein. The strain gaugetransducers are electrically connected in a bridge circuit to provide anoutput signal which varies proportionally with changes in the internalresistances thereof and thus with changes in the applied load.

In practice, the output signal varies non-linearly with applied load.There are at least three reasons for this nonlinearity. First, theapplied force results in an actual physical change in thecross-sectional area of the load cell. Second, the electricalcharacteristics of the bridge are non-linear because it is not possibleto realize a perfect constant current in the arms thereof. Third, thephysical properties of the strain gauges are non-linear and temperaturesensitive.

One approach in'the prior art to compensate for this non-linearity hasbeen the mounting of at least one additional strain gauge on thedeformable member so as to be responsive to changes in the strainloading thereof, and the electrical connection of this additionalcompensating strain gauge in circuit with either the input or outputdiagonals of the bridge circuit. That is, the compensating strain gaugeis connected externally of the bridge circuit, either in circuit withthe input leads from a bridge power supply, or in circuit with theoutput leads to an indicator therefor.

In operation, the compensating transducer reduces non-linearity in thebridge output signal by directly modifying the exciting signal suppliedto the bridge, or the output signal therefrom.

This approach is disadvantageous in that the circuit connection of thecompensating transducer, and its component value, must be changed forvaried applications of the load cell. Thus, one connection and componentvalue is required for a low-impedance power supply, another for ahigh-impedance power supply, yet another for a low-impedance indicator,and still another for a high-impedance indicator.

It is therefore an object of this invention to provide a load cell whoseoutput signal varies linearly with changes in applied load. I

It is a further object of this invention to provide such a load cellwhose bridge circuit is compensated for nonlinearity, notwithstandingthe impedance or type of input and output connections thereto.

SUMMARY OF THE INVENTION therein, and further including a matchingelement connected in an adjacent arm of the bridge circuit.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can perhaps best beunderstood by consideration of the following portion of thespecification, taken in conjunction with the accompanying drawings, inwhich:

FIG. I is a pictorial diagram showing a typical column type load celland the manner in which the strain gauges are mounted thereon; and

FIG. 2 is a schematic diagram of the load cell bridge circuit of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT With particular reference now toFIG. 1, the load cell includes a first portion 10, of generallycylindrical configuration, having a surface 10a which is sensitive toload strain resulting from an applied force F. The load cell alsoincludes a second portion 12, also of generally cylindricalconfiguration having a surface 12a which is not sensitive to loadstrainresulting from the applied force F. The load cell is supported ona stationary surface 14, and a force F, equal and opposite to force F,acts thereon.

In accordance with well-known practice, first and second compressionstrain gauges R R mounted on surface 10a in diametrically-opposingpositions, as are first and second tension strain gauges R R In normalpractice strain gauges R R R and R are connected in a bridge circuithaving input and output terminals. An exciting voltage is applied from apower supply across the input terminals thereof, and an output signal isderived from the exciting voltage by the transducers and appears acrossthe output terminals thereof. This output signal results from changes inthe internal resistance of each strain gauge due to changes in theapplied load F. The strain gauges are normally metallic conductorshaving a resistance change to strain (gauge factor) of 2 to 4.

To compensate for non-linearity, a preferred embodiment utilizesnon-linear strain gauge R mounted on surface 10a so as to be sensitiveto tension forces and connected in one arm of the bridge. This straingauge may comprise a semiconductor type due to advantages afforded by ahigh gauge factor (approximately 50 times that of metallic conductors)and an inherent non-linear resistance change with strain.

With particular reference to FIG. 2, a powersupply produces an excitingvoltage V, which is applied by input leads l6 and 20 to input terminalsor nodes 18, 22 of the bridge circuits. An output signal appears acrossthe output terminals or nodes 24, 30 thereof, and is coupled by leads 26and 3.2 to an appropriate indicator .means or control circuitry, notshown. Strain gauge R is connected from input node 18 to output node 24,and strain gauge R is connected from input node 22 to output node 24.The arm of the bridge from input node 18 to output node 30 includes theseries connection of strain gauge R and compensating nonlinearstraingauge R The arm of the bridge from input node 22 to output node 30includes the series connection of strain gauge R and a matching elementfor strain gauge R which, in FIG. 2, is shown to be a second non-linearstrain gauge R With reference back to FIG. 1, strain gauge R is mountedon surface 12a and thus is not sensitive to strain as a result of theapplied load F.

The normal strain gauge bridge includes an output node having a junctionof strain gauges R and R In accordance with the present invention, thiscommon node is split into two separate nodes 28a, 28b, by theintroduction of the compensating non-linear strain gauge, and itsmatching element in the bridge.

The configuration in FIG. 2 as an example uses the normally non-linearproperties of a semiconductor strain gauge to compensate for bridgenon-linearities. Therefore, since the compensating semiconductor straingauge R is mounted on the surface 100 the change in resistance thereofvaries non-linearly with strain, and thus with changes in the appliedload F. The matching element, such as R must be chosen to have a nominalvalue equal to that of R when no load F is being applied to the loadcell. In this manner, only the non-linear variations of R areeffectively seen by the bridge circuit. If desired, R may be of asemiconductor type identical to that of R to compensate fortemperature-caused non-linearities in the bridge.

In order to fully compensate for non-linearities in the bridge outputsignal, first and second trimming resistors R R may be placed in shuntwith semiconductor strain gauges R and R respectively. By appropriatechoice of the types and nominal values of the semiconductor straingauges R R and by appropriate adjustment of trimming resistors R and Rcompensation can be effected.

Alternative embodiments would include the bonding of the compensatingnon-linear strain gauge to surface 10a so as to be responsive tocompression forces resulting from the applied load F and the connectionof the compensating transducer and its matching element in other arms ofthe bridge.

In any case, the placement of the compensating semiconductor straingauge within the bridge allows the bridge to be used with practicallyany configuration or power supply or indicator, as neither the bridgeexcitation nor the output signal are directly influenced by the actionof the compensating transducer. Since the effect is symmetrical, thecompensation technique will also apply with the force (F) opposite tothat shown in FIG.

While this invention has been described with respect to a preferredembodiment, it is to be clearly understood by those skilled in the artthat the invention is not limited thereto, but rather is intended to bebounded only by the limits of the appended claims.

I claim as my invention:

1. A load cell for measuring an applied force comprising:

a. a member having a first surface deformable as a result of saidapplied force, and a second surface which is not so deformable,

b. a plurality of transducers mounted on said first surface,

c. a compensating non-linear strain sensitive element mounted on saidfirst surface,

(1. a matching element for said compensating nonlinear strain sensitiveelement mounted on said second surface, and

' e. means connecting said plurality of transducers into a bridgecircuit having a plurality of arms, and further including meansconnecting said compensating non-linear strain sensitive element in oneof said arms and said matching element in an adjacent one of said arms,whereby said bridge circuit produces an output signal which varieslinearly with changes in said applied force.

2. A load cell as recited in claim 1, wherein said compensatingnon-linear strain sensitive element comprises a semiconductortransducer.

3. A load cell as recited in claim 2, wherein said matching elementcomprises a semiconductor transducer.

4. A load cell as recited in claim 3, wherein said matching element isof the same type and nominal value as said compensating semiconductortransducer.

5. A load cell as recited in claim 1, further including first and secondtrimming elements connected in shunt with said compensating non-linearstrain sensitive element and said matching element, respectively.

6. A load cell as recited in claim 5, wherein said first and secondtrimming elements comprise variable resistors having the same nominalvalue.

7. A load cell as recited in claim 1, wherein one of said plurality oftransducers, and said compensating non-linear strain sensitive element,are mounted on said first surface so as to be sensitive to axial forcesexerted on said member, and further including means connecting said oneof said plurality of transducers and said compensating non-linear strainsensitive element in series in one arm of said bridge circuit.

8. A load cell as recited in claim 1, wherein one of said plurality oftransducers, and said compensating non-linear strain sensitive element,are mounted on said first surface so as to be sensitive to lateral (ortransverse) forces exerted on said member, and further including meansconnecting said one of said plurality of transducers and saidcompensating non-linear strain sensitive element in series in one arm ofsaid bridge.

is s a t t V UNITED STATES PATENT @TTTQT ETH ICATE E T'LEQTWN Patent No.3,759,093 Dated September 18, 1973 Inventor(s) Emory rr It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

On the cover sheet insert [72 Assignee Electro Development Corporation,Lynnwood, Washington Signed and sealed this 26th day of February 1971;.

(SEAL) Attest:

EDWARD MFLETCHEBJR. C. MARSHALL DANN- Attesting Officer Commissioner ofPatents FORM PO-105O (10-69) USCOMM'DC 50376-P59 fi' LLS. GOVERNM ENTPRINTING OFFICE I969 0-366-334,

1. A load cell for measuring an applied force comprising: a. a memberhaving a first surface deformable as a result of said applied force, anda second surface which is not so deformable, b. a plurality oftransducers mounted on said first surface, c. a compensating non-linearstrain sensitive element mounted on said first surface, d. a matchingelement for said compensating non-linear strain sensitive elementmounted on said second surface, and e. means connecting said pluralityof transducers into a bridge circuit having a plurality of arms, andfurther including means connecting said compensating non-linear strainsensitive element in one of said arms and said matching element in anadjacent one of said arms, whereby said bridge circuit produces anoutput signal which varies linearly with changes in said applied force.2. A load cell as recited in claim 1, wherein said compensatingnon-linear strain sensitive element comprises a semiconductortransducer.
 3. A load cell as recited in claim 2, wherein said matchingelement comprises a semiconductor transducer.
 4. A load cell as recitedin claim 3, wherein said matching element is of the same type andnominal value as said compensating semiconductor transducer.
 5. A loadcell as recited in claim 1, further including first and second trimmingelements connected in shunt with said compensating non-linear strainsensitive element and said matching element, respectively.
 6. A loadcell as recited in claim 5, wherein said first and second trimmingelements comprise variable resistors having the same nominal value.
 7. Aload cell as recited in claim 1, wherein one of said plurality oftransducers, and said compensating non-linear strain sensitive element,are mounted on said first surface so as to be sensitive to axial forcesexerted on said member, and further including means connecting said oneof said plurality of transducers and said compensating non-linear strainsensitive element in series in one arm of said bridge circuit.
 8. A loadcell as recited in claim 1, wherein one of said plurality oftransducers, and said compensating non-linear strain sensitive element,are mounted on said first surface so as to be sensitive to lateral (ortransverse) forces exerted on said member, and further including meansconnecting said one of said plurality of transducers and saidcompensating non-linear strain sensitive element in series in one arm ofsaid bridge.